In preparing for the role of Lt. Kozenczak, Michael Riley interviewed the real Joe Kozenczak at length to learn about the case and the man.
According to Brian Dennehy, John Wayne Gacy wrote him a letter in protest of the film and to proclaim his innocence. Dennehy did not respond to him.
While John Wayne Gacy (Brian Dennehy) continuously makes fun of Lieutenant Kozenczak's Polish heritage throughout the film, Gacy himself was actually of Polish descent (his paternal grandparents' original name was Gaca and sometimes spelled Gatza).
Like John Wayne Gacy, Brian Dennehy was a very burly man. However, in real life, Dennehy was much taller than the real Gacy. Gacy was only 5'8", while Dennehy stood at a solid 6'2".